Method of producing a metallic coating on magnesium and its alloys



Patented Oct. 17, 1950 METHOD OF PRODUCING A METALLIC COATING ONMAGNESIUM AND ITS ALLOYS Herbert K. De Long, Midland, Mich, assignor toThe Dow Chemical Company, Midland, Mi ch a corporation-gof'Delaware V HNo Drawing. Application October 6, 1947, Serial No. 778,283

9 Claims.

The invention relates to the surface treatment of articles of magnesiumand magnesium base alloyscontaining upwards of 85 per centof magnesium.I Itmore particularly concerns a surface treatment method for-theaforesaid metals which promotes the adherence thereto'of metalliccoatings, particularly those produced by electrodeposition.

Articles of magnesium and its alloys are among the most difiicult ofallcommon structural metals to provide with an adherent metallic coating ofanother metal. A number of methods have'been proposed heretoforeby'which coatings of certain metals maybe provided on articles ofmagnesium and its alloys. Howeveigthe coatings obtained are not whollysatisfactory either from the standpoint .ofiadhesion, appearance,orprotection afforded the underlying metal, and, in some instances, thecoating procedures are complex and difficult to perform. It is adesideratum in the art to provide a method of treatment by which amagnesium or magnesium-base alloy article maybe given an adherentcoating of another metal, and particularly an adherent metallic coatingwhich is either or both protective and decorative.

Accordingly, it isthe principal object of the invention to .provide 'amethod fulfilling this need. Aparticular object is to provide a methodof treating the surface of an article of a magnesium or magnesium-basealloy, whereby adherent electroplates of other metals thereon may beobtained. Other objects and advantages of the invention will appear asthe description of the invention proceeds.

Briefly the method contemplated by the invention comprises treating thecleaned article of magnesium or magnesium-base alloy with analkaline-aqueous solution containing pyrophosphate and zinc is solution,whereby a zinc coating is depositedupon the metal. The zinc coatedsurface obtained is used as a base for theretention of other metalliccoatings particularly those deposited thereon by electrodeposition, asfrom an alkaline cyanide aqueous. electroplating bath. Theseelectroplates may be used as the base for still other metallic coatings,.such as those which cannot be deposited directly on zinc or magnesium.

.Priorto carrying out the method, the surface of the articleis cleanedin a manner to leave the surface free from contamination and in acondition whereby the metal will react with the zincing solution, ofdissolved pyrophosphate and zinc, to bring about the deposition of thezinc coating. The surface of the article may be pre- 2 pared for zincingby a mechanical cleaning such as machining, wire brushing, buffing, andthe like, or by a chemical treatment with solvents or .pickling agents,the method used beingchosen .in accordance with the nature of thesurface to be cleaned and smoothness desired. While mechanical cleaningusuallyleaves the surface in a condition capable of reacting with thezincing solution, chemical treatment, such as the conventional picklingoperationsleavean oxidic film which interferes with .the action of thezincingsolution. 1

I have found that the oxidic film, however formed, should be removedbefore proceeding with the method of the invention. For the removal ofthe film or coating, the article may be immersed in a film removingsolution such as a 2 per cent solution of acetic-acid in Water or a 1percentsolution of hydrochloric-acid in-water. Still bet ter results asregards the adhesion of the zinc 1 coating subsequently to be appliedare obtained an article of magnesium and its alloys in' an aqueoussolution of aluminum chloride has ,spe-- cial advantages and is morefully set forthin my copending application, Serial, No. 778,282, filedOctober 6, 1947 issued'as Patent No.'2,499,269 on February 28, 1950.

In carrying out the method of the invention, the article to be zinccoated is treated with an aqueous zincing bath having a pH between about8 to 11 and comprising zinc and pyrophosphate is solution as aforesaid,as by immersing the ar-.

ticle in the bath, the article and treating solution being maintained incontact until a, coating of metallic zinc is deposited on the surface ofthe article. Formation of the coating is visually evident by its uniformgrey color and appears to result from the reducing action of themetallic magnesium of the surface of the article on the dissolved zincin the zinc-pyrophosphate solution, whereby the reduced zinc isdeposited in situ, the reduction proceeding'according to the:

A number of formulations of the ziric-pyrophosphate bath have been foundeffective to The aluminum deposit the zinc coating of which thefollowing are illustrative:

Example 1 Grams Sodium pyrophosphate (NGAPZO'!) 10 Zinc fluoride (ZnF2)1.5

Water sufficient to make 100 grams of solution The pH of the solution isabout 9.6.

Example 2 Grams Sodium pyrophosphate 10 Zinc sulfate (ZnSOa'lHzO) 4Sodium fluoride (NaF) 0.5

Water sufficient to make 100 grams of solution The pH of the solution isabout 10.2.

Example 3 Grams Sodium pyrophosphate 10 Zinc fluoride 1 Ferricfluoride(FeFa) 0.01

Water suflicient to make 100 grams of solution I The pH of solution isabout 9.8.

Example 4 Grams Potassium pyrophosphate (K4P207) 10 Zinc fluoride 0.5Sodium fluoride 2 Water sufficient to make 100 grams of solution The pHof the solution is about 9.6.

Example 5 Grams Sodium pyrophosphate 6 Zinc cyanide (Zn(CN)2) 1 Sodiumfluoride 2 Water sufficient to make 100 grams of solution The pH of thesolution is about 10.6.

Water su fficient to make 100 grams of solution The pH of the solutionis about 10.6.

.-The foregoing solutions are used preferably at their boilingtemperature, the article to be treated being immersed therein for about1 to 30 minutes or longer, although 3 to 7 minutes immersion ispreferred. The coating obtained is generally not over 0.0001 inch thickand cannot be peeled off.

Other alkali metal pyrophosphates may be used, such as those of lithium,rubidium, and cesium. If desired, the pyrophosphate in the bath may beprovided by adding pyrophosphoric acid to the bath and suflicient alkaliof the alkali metal family to raise the pH to the alkaline side, therebyin eifect producing an alkali metal pyrophosphate in solution as anactive ingredient. The zinc in solution may be derived from varioussoluble zinc compounds, in addition to those in the foregoing examples,such as zinc acetate, zinc borate, zinc bromide, and zinc chloride.

The concentration of the pyrophosphate and zinc in solution, effectivefor producing the Zinc coating, does not appear to be critical and awide variation in their concentrations is permissible. In the case ofthe pyrophosphate, the concentration of P201 may be from about 0.6 to27.0 grams, and the concentration of Zn from about 0.1 to 4.0

4 grams per 100 grams of solution, although other proportions may beused. Pyrophosphoric acid and various alkalis, such as an alkali metalhydroxide or carbonate, may be used to adjust the pH of the zinc platingsolution to the desired value, if necessary.

While the presence of dissolved fluoride, derived from a soluble metalfluoride (preferably an alkali metal fluoride), in the Zinc plating bathis preferred because it reduces the time of treatment and. results inbetter adhesion of the zinc, its presence is not essential and may beomitted, if desired, provided the bath contains the ions ofpyrophosphate and zinc and the pH is between about 8 and 11. A suitablefluorine concentration is between about 0.25 and 8 per cent, althoughother concentrations may be used.

Although the pyrophosphate-zinc bath is used preferably while at orabout the boiling point, lower temperatures may be used but the rate ofzinc deposition is slower. At 20 to 25 C., for example, a zinc coatingis obtained which appears to reach a maximum thickness, in about 30minutes; while at the boiling point or near 100 C., a similar coatingisobtained in about 1 minute.

In forming the coating, there is a mild gassing at the surface of theimmersed article especially at first and this gassing diminishes as thecoating action proceeds. On attaining its ultimate thickness, thereaction appears to cease and little or no gassing is observed. Thearticle need not be left in the bath beyond the stage when gassingceases or nearly ceases,

Another indication that the Zinc coating op-v eration has proceeded to alimiting extent is that the surface undergoing plating becomes uniformlygrey in appearance. It is to be understood that useful coatings areobtained in less time than that required for the coating action to cometo an apparent end, as when gassing ceases, or nearly ceases, or whenthe article becomes uniformly grey.

Immersion of the article in the. zinc plating solution is the preferableway of maintaining contact between the solution and. the surface to becoated, but other ways of maintaining contact may be used such asspraying the solution onto the surface of the article. After thetreatment, the plated article is thoroughly rinsed in Water.

The zinc coating obtained by the foregoing method, referred tohereinafter sometimes as a contact coating, has utility as a protectionand decoration, and, if desired, the coated article need not be furthertreated except to dry it. However, the zinc contact coatin has theunique advantage of being capable of receiving adherent deposits ofother metals in similar manner to that of zinc itself. For example, thecontact coating before drying may be electroplated in conventionalcyanide alkaline plating baths suitable for electroplating zinc. Variousmetals may be thu deposited from their alkaline'electroplating baths, e.g. silver, copper, gold, cadmium, tin, and various combinations of thesemetals. 1

In providing an electroplating over the contact I coating, the coatedarticle is preferably given a strike from a cyanide platin bath, forexample, a copper strike from an alkaline copper plating bath, such as aconventional copper cyanide.

Grams Copper cyanide (CuCN); 41.3 Sodium cyanide (NaCN)' 50.8 ,Sodiumcarbonate (NazC'O'sl 30.0 Rochelle salt (KNaCzH4O64H2O') 45.0

Other conventional alkaline cyanide plating baths may be used,suchasgthose of theaforementioned. metals, althoughqforz the purposeofobtaining a strike, the copper plating bath is preferred. In forming:a:.copp e deposit using the above bath, it ,isoperated,.preferablyj at atemperature of about 150 F. A currentdensity of about 5m 10. amperes;per square. footof surfacewis maintained .for about .3 minutesorlongerand then raised to about 15 to 20 amperes per square: foot for 2 minuteslonger. This treatment results in coating the zinced surface with alayer of copper that adheres well and covers -tl e zin c. If desired,the copper plating operation may be continued for a longer time toproduce a heavier coating and other current densities andplatingtemperature may be used as known in the electroplatingart.

Having applied a strike of copper or other metal coating .over thecontact coating, it is possible tov further. electroplate the articlewith any other metal that is. capableof being electrodeposited upon thestrikecoating. Fo example, I have found that cadmium, chromium nickel,silver, and zinc or combinations of metals can be electroplatedinconventional manner on the electroplated'contact coating.

Among the advantages-of the method are that by either contact coatingthe magnesium (or magnesium alloy) article with zinc or electroplatingthe zinc contact coated article, it is possible to provide a finishcoating of any metal that is capable of electrodeposition from aconventional plating bath. As already indicated, for example, thecyanide plating bath may be used for electroplating over zinc (depositedby contact coating or plating as described or electrolytically over thecontact coating) an of the various metals that can be plated on zincfrom such baths. Furthermore, by choosing as the metal to be depositedfrom the cyanide bath one over which a plating may be deposited from anacid plating bath, for example, the metal copper, all the metals whichare capable of being applied as electroplates from either a conventionalacid or alkaline plating bath, may be deposited upon articles ofmagnesium and its alloys as a finish coating.

For example, metals such as chromium and nickel which cannot be plateddirectly on magnesium or zinc, because of the acid nature of theirplating baths, can be readily electroplated as a finish coating on amagnesium and magnesium alloy article from the conventionalelectroplating baths of these metals after the article is zinc contactcoated according to the invention and plated from an alkaline bath witha metal resistant to acid plating baths.

I claim:

1. In a method of forming an adherent metallic coating upon the surfaceof an article of magnesium and its alloys, the step which comprisestreatin the surface of the article with an al- .mide, zinc chloride,zinc cyanide, zinc fluoride,

zinc sulfate, the concentration of the pyrophosphate radical beingbetween 0.6 and 27 grams and that of the. zinc being between 0.1 andligrams per I00 grams of .the solution, whereby a coating of'metalliczinc is deposited in situ on the surface of the article.

2. In a method of forming an adherent metallic coating upon the surfaceof. an article .of magnesiumand its alloy the step which comprisestreatin the surface ofithe article withan alkaline aqueous solutionhaving a pH. between about '8' and l1 and consisting essentially ofwater, an. alkali metal pyrophosphate, and a water-soluble zincsaltfiselectedj from the group consisting of zinc acetate, zinc ,borate,zinc bro.- mide, zinc chloride, zinc cyanide, zinc, fluoride, zincsulfate, the concentration. of the pyrophosphate radical being; between0;6' and 27 grams and thatof flthezinc being between 0.1 and'4 grams pergrams of the solution and, the temperatureof the-solutionbeing betweenabout 20 C. andits atmospheric boiling point, whereby a coatingofmetallic,zinc-isdepositedin situ on the, surface of the. article.

3'. In a method of forming an'adherent'metallic coating upon the.surface of. an. articlev of magnesiumand its. alloys, the stepwhicncomprises treatingthe surface of the, article. with an, alkalineaqueous solution. having,a pI-Ii be.-

twe'enaboutfif.ancLll and consisting essentially of water, an alkali.metal pyrophosphate, a water-soluble. zinc. salt selfectedfromvv the.group consistin of zinc. acetate; zincborate, zincbronide, ,zinc.chloride, zinc cyanide;,.zinc. sulfate, the concentration of thepyrophosphate radical being between 0.6 and 2'7 grams and that of thezinc being between 0.1 and 4 grams per 100 grams of the solution, and awater-soluble metal fluoride in amount between about 0.25 and 8 gramsper 100 grams of the solution, whereby a coating of metallic zinc isdeposited in situ on the surface of the article.

4. In a method of forming an adherent metallic coating upon the surfaceof an article of magnesium and its alloys, the step which comprisestreating the surface of the article with an alkaline aqueous solutionhaving a pH between about 8 and 11 and consisting essentially at water,an alkali metal pyrophosphate in amount sufiicient to furnish 0.6 to 27grams of pyrophosphate radical per 100 grams of the solution, zincsulfate in amount sufficient to furnish 0.1 to 4 grams of zinc per 100grams of the solution, and a water-soluble metal fluoride in amountsuflicient to furnish between 0.25 and 8 grams of fluoride per 100 gramsof solution.

5. In a method of forming an adherent metallic coating upon the surfaceof an article of magnesium and its alloys, the step which comprisestreating the surface of the article with an alkaline aqueous solutionhaving a pH between about 8 and 11 and consisting essentially of water,an alkali metal pyrophosphate in amount sufficient to furnish 0.6 to 27grams of pyrophosphate radical per 100 grams of the solution and zincfluoride-in amount suflicient to furnish 0.1 to 4.0 grams of zinc per100 grams of solution,

whereby a coating of metallic zinc is deposited in situ on the surfaceof the article.

6. In a method of forming an adherent metallic coating upon the surfaceof an article of magnesium and its alloys, the step which comprisestreatin the surface of the article with an alkaline aqueous solutionhaving a pH between about 8 and 11 and consisting essentially of water,an alkali metal pyrophosphate in amount suflicient to furnish 0.6 to 27grams of pyrophosphate radical per 100 grams of the solution, zincfluoride in amount sufficient to furnish 0.1 to 4.0 grams of zinc per100 grams of solution, and ferric fluoride in amount sufficient tofurnish 0.25 to 8 grams of fluoride per 100 grams of solution, whereby acoating of metallic zinc is deposited in situ on *the surface of thearticle.

7. In a method of forming an adherent metallic coatin upon the surfaceof an article of magnesium and its alloys, the step which comprisestreating the surface of the article with an alkaline aqueous solutionhaving a pH between about 8 and 11 and consisting essentially of water,an alkali metal pyrophosphate in amount sufficient to furnish 0.6 to 27grams of pyrophosphate radical per 100 grams of the solution, zincfluoride in amount sufiicient to furnish 0.1 to 4.0 grams of zinc per100 grams of solution, and. sodium fluoride in amount suflicient tofurnish 0.25 to 8 grams of fluoride per 100 grams of solution, whereby acoating of metallic zinc is deposited in situ on the surface of thearticle.

8. In a method of formin an adherent metallic coating upon the surfaceof an article of magnesium and its alloys, the step which comprisestreating the surface of the article with an alkaline aqueous solution.having a pH between about 8 and 11 and consisting essentially of water,sodium pyrophosphate in amount sufficient to provide 0.6 to 27 grams ofpyrophosphate radical per 100 grams of the solution,

zinc cyanide in amount sufficient to provide 0.1 to 4.0 grams of zincper grams of the solution, and sodium fluoride in amount suflicient toprovide 0.25 to 8 grams of fluoride per 100 grams of the solution,whereby a coating of metallic zinc is deposited in situ on the surfaceof the article.

9. In a method of forming an adherent metallic coating upon the surfaceof an article of magnesium and its alloys, the step which. comprisestreating the surface of the article with an alkaline aqueous solutionhaving a pH between about 8 and 11 and consistin essentially of Water,sodium pyrophosphate in amount sufficient to furnish 0.6 to 2'7 grams ofpyrophosphate radical per 100 grams of the solution, zinc cyanide inamount sufficient to furnish 0.1 to 4 grams of zinc per 100 grams of thesolution, sodium fluoride in amount 'sufiicient to furnish 0.25 to 8grams of fluoride per 100 grams of the solution, and ferrous oxalate,whereby a coating of metallic zinc is deposited in situ on the surfaceof the article.

HERBERT K. DE LONG.

REFERENCES CITED The following references are of record in th file ofthis patent:

UNITED STATES PATENTS Number Name Date 744,170 Darlay Nov. 17, 19031,607,676 Jirotka Nov. 23,1926 1,627,900 Hewitson May 10, 1927 1,801,629Kenaga Apr. 21, 1931 2,297,241 Perner Sept. 29, 1942 2,313,756 LooseMar. 16, 1943 2,419,190 Wagoner Apr. 15', 1947 OTHER REFERENCES IronAge, September 20, 1888, page 42

1. IN A METHOD OF FORMING AN ADHERENT METALLIC COATING UPON THE SURFACEOF AN ARTICLE OF MAGNESIUM AND ITS ALLOYS, THE STEP WHICH COMPRISESTREATING THE SURFACE OF THE ARTICLE WITH AN ALKALINE AQUEOUS SOLUTIONHAVING A PH BETWEEN ABOUT 8 AND 11 AND CONSISTING ESSENTIALLY OF WATER,AN ALKALI METAL PYROPHOSPHATE, AND A WATER-SOLUBLE ZINC SALT SELECTEDFROM THE GROUP CONSISTING OF ZINC ACETATE, ZINC BORATE, ZINC BROMIDE,ZINC CHLORIDE, ZINC CYANIDE, ZINC FLUORIDE, ZINC SULFATE, THECONCENTRATION OF THE PYROPHOSPHATE REDICAL BEING BETWEEN 0.6 AND 27GRAMS AND THAT OF THE ZINC BEING BETWEEN 0.1 AND 4 GRAMS PER 100 GRAMSOF THE SOLUTION, WHEREBY A COATING OF METALLIC ZINC IS DEPOSITED IN SITUON THE SURFACE OF THE ARTICLE.